When running a casing or liner into a predrilled bore hole, it is desirable that the bore hole will have been drilled with the intended shape, to its designed diameter, and without marked deviations, such as doglegs, along its path. Unfortunately, due to unstable, heterogeneous formations, irregularities such as stringers within a formation, poor drilling practices, damage and wear of drill bits and bottom hole assemblies (BHA) and various other factors, the ideal bore hole is rarely achieved.
Therefore, it is desirable to provide the casing or liner string being run into the existing bore hole with a cutting structure at the leading end thereof to enable enlargement, as necessary, of portions of the bore hole so that the casing or liner may be run smoothly into the bore hole to the full extent intended. Initially the entire liner or casing string was rotated while it was being lowered into the borehole, which required powerful and complex drive systems at the surface. More recent projects use a hollow turbine or motor at the leading end of the casing string which are driven by drilling fluid pumped from the surface. It provides for a more efficient and economical transfer of power from the surface to the drill bit but it also limits the amount of torque that can be delivered to the bit and most of the power is in the form of high rotational speed. This most recent approach of using high speed turbines to provide a casing or liner string with a reaming capability has yielded inconsistent results with conventional, bullet shaped reaming bits.
In U.S. Pat. No. 7,621,351 a reamer bit having a substantially tubular body and a nose portion with a concave center extends from the nose portion to the side wall through a tapered shoulder region. The reaming tool further comprises a cutting structure for enlarging, also termed “reaming,” of a bore hole through contact with the side wall thereof. The term “tool” is used herein in a non-limiting sense, and embodiments of the present invention may also be characterized as a reaming bit or reaming shoe. In some embodiments, the nose portion of the reaming tool has at least one port therethrough extending to the interior of the body. In some embodiments, a plurality of circumferentially spaced, spirally configured blades extend on the exterior of the body from proximate the shoulder transition region to the gage and define junk slots there between. An axially leading end of each blade commences with substantially no standoff from the body and tapers radially outwardly to a portion having a substantially constant standoff and having a radially inwardly extending, beveled, axially trailing end. A plurality of cutting elements are disposed along a rotationally leading edge of each blade. The nose of this tool can be drilled out in a related method to allow further completion of the well.
In the past reaming tools that were surface driven turned typically in an RPM range of about 40-80 RPM and the large diameter, stiff casing was able to transmit high levels of torque. Turbines or high speed motors driven at speeds of 300-600 RPM and higher can only supply a fraction of the torque provided by top drives or rotary tables. Due to the lower torque capacity of the turbines the reaming tools that were previously serviceable experienced a great deal of stalling, reduced rates of penetration and generally unreliable performance. Typically these reamers had a bullet shaped profile 10, shown in FIG. 1, from the cylindrical gage dimension 12 to the center 14 of the concave cone section 16 that featured a long tapered segment 18 sandwiched between a curved segment 20 that had one or two radii 22 and a lower curved transition 24 having a radius 26 that forms the leading part or nose of the profile and then continues in a bottom taper 28 that defines a recessed, concave cone 16. In FIG. 1 the profile length (PL) is defined as the distance along the profile between and not including the gauge dimension 12 and the center 14 of the cone 16. The nominal diameter or bit size (BS) is double the distance from the centerline 30 to the gauge dimension 12 in a plane perpendicular to the centerline 30. The range of PL/BS ratios of existing reamer tools that were run in the typical RPM range of 40-80 RPM was in the order of 0.76 to 1.27 for a range of BS of 5.5 to 19.25 inches. In addition to the profile length the inclination α of the long tapered section with respect to the reamer axis 30 is important. It forms a conical wedge in the borehole which provides a mechanical advantage by producing high lateral forces for small changes in axial forces or weight on bit (WOB). The mechanical advantage is proportional to 1/tanα and therefore is quite significant for smaller angles. This is desirable in applications where it is difficult to deliver sufficient WOB to advance the reamer but becomes the source of high torsional oscillations in applications where WOB control is difficult or erratic due to a complex well trajectory, borehole tortuosity, formation heterogeneity and many other operational variables.
While the various reamers described above functioned fairly well at higher torque and slower RPM, the recent advent of a turbine driving a reamer with less torque at significantly higher speeds of 300-600 RPM and above produced an unacceptable level of torque fluctuation and stalling of the turbines. The present invention was developed to address this situation and enhance the performance of reamers in turbine applications by making modifications to the profile and other design features as will be described below. One of the approaches was the profile modification and shortening of the PL by using a plurality of arcuate surfaces between the gauge dimension 12 and bottom taper 28 and eliminating the long, low angle, tapered segment 18 of FIG. 1. Another variation was to retain it but reduce its length and increase the angle of the tapered segment 18 to more than 30 degrees which reduces the aggressiveness and brings the PL/BS ratio to below 0.75. A different source of undesirable vibrations and torsional oscillations at low torque and high rpm is a perfectly symmetrical spacing of blades. Even small variations in the angular spacing between blades will significantly reduce these harmonic vibrations without having to affect the mass balance of the reamer itself. Another feature to assure reliable performance of the reamer was to extend the reamer blades into the concave cone section 16 and add additional fluid ports to enhance bottomhole cleaning. Thus the reamer is capable to effectively drill a full diameter borehole in case the pilot hole gets completely obstructed, is irregularly shaped or is backfilled with cave-ins and/or a cuttings bed in inclined, extended reach wells. Other features were added to the blade structure to protect the outer casing when running the casing or liner string through an already cased upper hole section. The long, spiraled gage pads which are extensions of the blades along the cylindrical section of the reamer bit are designed with smooth but highly wear resistant surfaces to minimize the borehole wall contact stresses and stabilize the bit at high speeds. The upper or trailing end of the gage pad is provided with a single row of active cutting elements for back-reaming while the casing string is moved up and down to condition the borehole and keep the reamer from getting stuck. At the transition from the gage pads to the leading, actively cutting blades the outer surface of the blades includes a peripheral step to allow greater exposure of the primary cutting elements. A series of projections rotationally behind the primary cutting elements limit the depth of cut to further control unintended weight on bit spikes, torsional oscillations and stalling in interbedded, mixed strength formations. These and other features of the present invention will be more readily apparent to those skilled in the art from a review of the detailed description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be determined from the appended claims.